1. Field of the Invention
The present invention relates generally to radiation therapy catheters, and, in particular, to an apparatus and method for properly centering or otherwise positioning a radiation therapy catheter within a vascular segment at a desired treatment site.
2. Description of the Related Art
Approximately 20-30% of patients who undergo arterial intervention experience restenosis within about 6 months of the initial treatment. This often necessitates repeating the procedure, such as balloon angioplasty, stent implantation, atherectomy, or treatment with lasers, to once again clear the patient""s vascular obstruction. Repeating such a procedure, or undertaking a second, different procedure, is clearly undesirable.
Although stent implantation is used to prevent restenosis, stent restenosis occurs due to neointimal proliferation, i.e. an accelerated growth of tissue at the treated site. However, endovascular radiation effectively inhibits neointimal formation. In particular, a radiation treatment may be undertaken either prior to placement of a conventional stent or through the use of a radioactive stent, i.e. a stent that is coated or impregnated with a radioactive source. The stent may be made radioactive, for example, by placing it in a cyclotron that emits radionuclides.
Radiation therapy undertaken during or after arterial intervention can be accomplished in a variety of ways, as discussed, for example, in U.S. Pat. Nos. 5,213,561 to Weinstein et al., 5,484,384 to Fearnot, and 5,503,613 to Weinberger. Among other radiation therapy devices, these references disclose a guidewire having a radioactive tip, a radioactive source within a balloon catheter, and a radioactive source mounted on a balloon expansible stent. Weinberger teaches that a variety of radiation sources may be used, such as pellets, a wire, or a paste.
One frequently encountered problem is the difficulty in controlling the amount of irradiation; although, a guidewire or catheter type delivery device provides greater control of exposure time than an implanted radioactive stent. Further, a stent should match the length of the vessel segment to be treated; whereas, a guidewire or catheter can be moved axially to increase the length of the vessel exposed to radiation. With a stent, there is also the possibility that the radioactive material will leach into the surrounding tissue, as well as the possibility of thrombosis forming on the stent wire as a delayed re-endotheliatization of the stent struts.
With respect to possible exposure of the clinician or patient to radioactive material, it is easier to control exposure with a guidewire or catheter device. A sleeve or the like suitable for shielding the radioactive element can be used until the element is located at the desired treatment site. A radioactive stent, on the other hand, requires handling prior to insertion into the patient, and can result in increased radiation exposure. Also, it can be even more hazardous to handle, inject, and withdraw radioactive fluid from a balloon catheter.
In previous devices, there is also the difficulty in positioning a device at the vessel segment identified for radiation therapy. U.S. Pat. No. 5,199,939 to Dake et al. attempts to address this problem by providing stiffening members within an otherwise flexible member. An axial arrangement of radioactive pellets at the distal end of the device delivers the radioactive dosage. U.S. Pat. No. 5,503,613 to Weinberger discloses a computer-controlled afterloader that accurately places the radiation delivery wire within the blind lumen, which is sealed at its distal tip. Among the inputs to the afterloader are the location of the vessel segment, the diameter of the treatment site, and the radioactive characteristics of the radioactive element.
Unless a radiation dose delivery wire is carefully centered within a blood vessel, a relatively high radiation zone is obtained at that segment of the vessel contacting or closest to the wire, and a lower radiation zone elsewhere. U.S. Pat. Nos. 4,998,932 to Rosen et al. and 5,566,221 to Smith et al. disclose the use of balloons that aid in the centering of a catheter within a vessel. A balloon must be adequately inflated so that it contacts the vessel walls without damaging tissue or rupturing. If inflating the balloon causes enlargement of the vessel at the treatment site, the increased radius diminishes the level of radiation reaching the vessel walls. For example, irradiating tissue to a depth of about 2-3 mm within the tissue is usually desirable. A change in the diameter of the vessel diminishes the accuracy of the coordinates used by an afterloader, and the radioactive material may lack the intensity required for the desired penetration. If the balloon ruptures, pieces of the balloon may be carried downstream.
Other uses of balloons in radiation delivery include a segmented balloon centering device (see Verin et al., xe2x80x9cIntra-arterial beta irradiation prevents neointimal hyperplasia in a hypercholesterolemic rabbit restenosis model,xe2x80x9d Circulation, vol. 92, pp. 2284-2290, 1995) and a helical balloon, which is said to provide better flow around the catheter (see R. Waksman, xe2x80x9cLocal Catheter-Based Intracoronary Radiation Therapy for Restenosisxe2x80x9d, The American Journal of Cardiology, vol. 78, p. 24, 1996).
Thus, there is still a need for a radiation delivering catheter that can be accurately and easily centered within a vascular segment.
The present invention satisfies the need for a device that can be accurately centered within a vessel to be radioactively treated. By accurately centering a radiation catheter within the vessel, the walls of the vessel to be treated, e.g. at a site of a stenosis, are exposed to radiation flux that is more uniform than it would be if the radiation catheter were in contact with (e.g., resting on) the vessel wall.
In the present invention, various means are utilized to properly position a radiation treatment device within the vessel. In one embodiment of the present invention, one or more balloons are employed to position a radiation delivering catheter within the vessel and away from the vessel wall. In another embodiment of the invention, expandable structures are used to do the same. In yet another embodiment of the invention, a radiation source residing within a balloon is shielded from the vessel walls when the balloon is not inflated, but exposes the vessel walls to radiation when the balloon is inflated.
In one embodiment, a self-centering radiation device for treating a segment of a vessel in a patient comprises a catheter for delivering radiation, a plurality of balloons for securing the radiation catheter within the vessel (in which at least two of the balloons are independently inflatable), and a radioactive source for treating the vascular segment, in which the radioactive source is in proximity with the radiation catheter and positioned near the balloons during treatment.
In another embodiment of the invention, a radiation device for treating a segment of a vessel in a patient comprises a catheter for delivering radiation, a radioactive source in proximity with the radiation catheter, a noncompliant balloon around the radioactive source, a lumen within the radiation catheter that is in fluid communication with the noncompliant balloon to permit inflation and deflation of the noncompliant balloon, and a compliant balloon that surrounds the noncompliant balloon. The compliant balloon expands as the noncompliant balloon expands to radioactively treat the vascular segment.
In another embodiment of the invention, a radiation device for treating a segment of a vessel in a patient comprises a catheter for delivering radiation, a radioactive source in proximity with the radiation catheter, a balloon around the radioactive source, and a lumen within the radiation catheter that is in fluid communication with the balloon, permitting inflation and deflation of the balloon. The balloon has strips of material thereon that substantially shield the surroundings from unwanted radioactive exposure when the balloon is not inflated, in which the area between the strips increases as the balloon expands to more directly expose the vascular segment to radioactive treatment.
Another embodiment of the invention is a self-centering radiation device for treating a segment of a vessel that comprises a catheter for delivering radiation, at least one expandable structure for securing the radiation catheter within the vessel, and a radioactive source for treating the vascular segment, in which the radioactive source is in proximity with the radiation catheter. The expandable structures may be self-expanding and the device may further comprise one or more sheaths for expanding and compressing the expandable structures.
The invention further includes methods of treating a segment of a vessel. One method comprises inserting a catheter into the vessel, inserting a plurality of balloons into the vessel (in which at least two of the balloons are independently inflatable), inflating at least two of the independently inflatable balloons to position the catheter away from the walls of the vessel, and exposing the vascular segment to radiation treatment.
Another method of treating a segment in a vessel comprises inserting a balloon into the vessel, placing the balloon near the vascular segment to be treated, and expanding the balloon to expose the vascular segment to radiation that is located in the balloon""s interior.
Yet another method of treating a segment in a vessel comprises inserting a catheter into the vessel, inserting at least one expandable structure into the vessel, expanding the structure to position the catheter away from the walls of the vessel, and treating the vascular segment by exposing it to radiation.
In the embodiments of this invention, radiation can be delivered to the stenosis site by, for example, bonding a radioactive source directly onto the radiation catheter, or by passing radioactive carriers through a lumen within the radiation catheter. For example, the radioactive carriers, which may be in the shape of cylinders or spheres, can be carried towards (or away from) the stenosis site by fluid that is forced into (or out of) the lumen.